The use of opto-isolators in electronic circuits has become quite common where electrical isolation between two circuit portions is desirable. Opto-isolators typically are made up of a light tight package including a light emitting diode and a light responsive transistor. The electrical separation is created by the transmission of information by the light from the light emitting diode to the light responsive transistor. In addition to providing electrical isolation, the opto-isolator has been used as extensively as an indicating or control device. This is true where the light emitting diode is energized in response to the energization of some other circuit portion so that the light responsive transistor can indicate or control further circuit elements. While the elements that make up an opto-isolator are normally quite reliable, there is always a possibility that the light emitting diode or the associated transistor could fail and the opto-isolator therefore become inoperative without that fact being known. In certain types of safety related equipment, such as flame safeguard equipment, the failure of an opto-isolator in a control or indicating circuit could be very serious. In recent years flame safeguard equipmemt utilizing microprocessors for control of the sequence of burner operations have been developed. Opto-isolators are used to convey information as to the status of relay contacts in the burner control system to the microprocessor. In this type of equipment the opto-isolators have been used for voltage isolation as well as for transmission of critical data. The failure of an opto-isolator in a critical circuit, such as a control circuit to a fuel valve, could cause an explosive situation at a burner or boiler.